In chronic renal disease, the portions of the kidney that are relatively undamaged are responsible for maintaining kidney function; yet deterioration often ensues, even in the absence of activity of the primary disease. Similarly, in experimental renal insufficiency induced by subtotal nephrectomy in the rat there is at first good compensation; later renal tissue deteriorates with the appearance of hypertension, azotemia, and glomerular permeability with increasing proteinuria. It is the objective of this study to obtain ultrastructural data, especially stereological data, concerning the glomerular changes following subtotal nephrectomy. We expect the stereological findings to give some insight into the pathological changes in glomerular hemodynamics, and by inference into permeability changes. Permeability changes are also being studied directly by a variety of probe molecule techniques combined with electron microscopy.